At present, liquid crystal display devices have found varieties of applications as a thin light color display. Among the color liquid crystal display devices, transparent liquid crystal display devices, equipped with a light source at the back, are particularly popular and their use is still expanding.
A reflective-type liquid crystal display device, since adopting a different display method from the transparent liquid crystal display device with no backlight, can save the power consumption for the light source. In the reflective-type liquid crystal display device, the space and weight for the backlight can be spared. In other words, the reflective-type liquid crystal display device consumes less power as a whole and can obtain enough power from even a smaller battery, being suitable for devices whose priority is light weight and thin size. If the reflective-type liquid crystal display device is designed having the same size or weight as the transparent liquid crystal display device, the reflective-type liquid crystal display device can accommodate a larger battery, and therefore operate much longer than the transparent liquid crystal display device.
Another feature of the reflective-type liquid crystal display device can be found in the contrast characteristics of the display surface. The light emitting display, for example a Cathode Ray Tube (CRT), has a greatly decreased contrast ratio in the daylight. A transparent liquid crystal display device having undergone low reflection processing also inevitably suffers from a largely decreased contrast ratio when the surrounding light, for example, direct sun light, is too strong in comparison to the display light. By contrast, the reflective-type liquid crystal display device provides display light proportional to the amount of surrounding light, being especially suitable for apparatuses that are often used outdoors, such as portable information terminals, digital cameras, and portable video cameras.
Despite very promising fields of applications as discussed above, no reflective-type liquid crystal display device has been successfully made so far with satisfactory operability due to insufficient performance regarding the contrast ratio, reflectance, full color display, and compatibility with high definition display and animation.
The following description will explain the reflective-type liquid crystal display device in more detail.
A conventional TN (twisted nematic) liquid crystal display device, including two polarizer plates, boasts an excellent performance in contrast ratio and viewing angle dependence, but inevitably has a low reflectance. Moreover, in the TN liquid crystal display device, since the liquid crystal modulation layer and the light-reflective layer are distanced by the thickness of a substrate and other components, there occurs a difference in the optical path when the incident light enters and reflects, causing a parallax.
A transparent liquid crystal display device typically displays colors with an arrangement (display mode) including a combination of a single liquid crystal modulation layer and a color filter that is painted in color such that neighboring pixels provide color elements and is arranged such that the neighboring pixels act s display units. However, if that display mode is applied to the reflective-type liquid crystal display device and light travels along a path that inclines with respect to the normal direction of the display surface, there is a possibility that the incident light and the reflected light, when travelling through the color filter, pass through different color elements. For these reasons, the application of the display mode to the reflective-type liquid crystal display device is not suitable for high resolution and fidelity in color, and the reflective color display using the display mode has not yet commercialized.
Meanwhile, a Guest-Host liquid crystal display device (hereinafter, will be referred to simply as a GH) using no or only one polarizer plate and containing dye in liquid crystal has been developed. However, the GH lacks reliability due to the addition of dye in the liquid crystal, and fails to offer a high contrast ratio due to a low dichroism of the dye. Especially, in color display using a color filter, color purity drops greatly due to insufficient contrast, and therefore a color filter having a high color purity needs be combined. The use of a color filter with high color purity, however, causes a decrease in brightness and erases off the advantage of this method that the high brightness is achieved without a polarizer plate.
This is the background leading to the development of liquid crystal display devices using a single polarizer plate (hereinafter, will be referred to as a single polarizer plate method) that is expected to achieve high resolution and high contrast display. Among such liquid crystal display devices, the largest number of disclosures are made on those combined with a quarter-wave plate that are expected to realize high contrast.
Japanese Laid-Open Patent Application No. 55-48733/1980 (Tokukaisho 55-48733/1980) discloses an example of such a liquid crystal display device: specifically, a reflective TN-method (45.degree.-twisted) liquid crystal display device using one polarizer plate and a quarter-wave plate. This prior art, using a 45.degree.-twisted liquid crystal layer and controlling the electric field applied across the liquid crystal layer, displays black and white by effecting two states in which the polarization plane of the incident linearly polarized light into the quarter-wave plate is parallel and twisted by 45.degree., respectively, to the optic axis of the quarter-wave plate. The liquid crystal cell includes a polarizer, a 45.degree.-twisted liquid crystal layer, a quarter-wave plate, and a reflective plate in sequent from the light entering side.
U.S. Pat. No. 4,701,028 (Clerc et al.) discloses a reflective-type liquid crystal display device including a combination of a single polarizer plate, a quarter-wave plate, and a vertically aligned liquid crystal cell. Japanese Laid-Open Patent Application No. 6-337421/1994 (Tokukaihei 6-337421/1994) discloses a reflective-type liquid crystal display device including a combination of a single polarizer plate, a quarter-wave plate, and a bend vertically aligned liquid crystal cell. A New Reflective Display with High Multiplexibility and Gray Scale Capability (Euro Display '96, page 464) also discloses a reflective-type liquid crystal display device including a combination of a single polarizer plate, a quarter-wave plate, and a vertically aligned liquid crystal cell.
SID 96 Digest (page 763) discloses an example of an application to a reflective projection of a display mode in which chiral-dopant-containing liquid crystal having negative dielectric anisotropy is sandwiched between upper and lower substrates having undergone vertical alignment processing.
The following description describes a display operation of the single polarizer plate method disclosed in above Japanese Laid-Open Patent Application No. 6-337421/1994.
The polarizer plate provided to the liquid crystal display device cell on its light entering side allows only the incident light and outgoing light polarized linearly in a certain direction to pass therethrough and blocks the light is polarized linearly in all the other directions. If no electric field is applied across the liquid crystal layer, the incident light having passed through the polarizer plate is converted into circularly polarized light by an optical retardation compensation plate such as a .lambda./4 plate (quarter-wave plate), enters the liquid crystal layer, passes through the vertically or substantially vertically aligned liquid crystal layer, and, without any further conversion or change, reaches a reflective plate. The light having reached the reflective plate is converted into circularly polarized light of reverse rotation by the reflective plate, passes through the liquid crystal layer, the .lambda./4 plate and other components in reverse order from the incident light, is converted to light that is linearly polarized vertical to the linearly polarized incident light. The dark state is thus effected.
In addition, if the liquid crystal inclines upon application of an electric field across the liquid crystal layer to realize a phase difference of certain conditions, the incident circularly polarized light having passed through the polarizer plate and .lambda./4 plate, is converted into linearly polarized light, becomes a linearly polarized line at the reflective plate, resulting in out-going linearly polarized light having the same polarization direction as the linearly polarized light, at the polarizer plate, which is the incident light for the display device having passed through the polarizer plate. The bright state is thus effected.
In other words, regarding the incident and outgoing light vertical to the liquid crystal display device, it is a public knowledge that the necessary and sufficient condition for realizing the bright state is that the light is polarized linearly in an arbitrary direction when it is reflected by the reflective plate, and that the necessary and sufficient condition for realizing the dark state is that the light is polarized circularly leftwards or rightwards when it is reflected by the reflective plate.
In addition, there are problems as follows with the vertical alignment liquid crystal display device described in U.S. Pat. No. 4,701,028, U.S. Pat. No. 4,492,432, Japanese Laid-Open Patent Application No. 6-337421/1994, and A New Reflective Display wit High Multiplexibility and Gray Scale Capability (Euro Display '96).
First, since the directions of the vertical alignment, especially those of the tilted vertical alignment, are parallel between the upper and lower substrates, the liquid crystal inclines in one direction and the viewing angle dependence on the display surface is extremely large. Secondly, since the reflectance has a large wavelength dependence, there occurs undesirable coloring.
As for the display mode described in previously mentioned SID 96 Digest (page 763), no quarter-wave plate is used, and a linearly polarized light is guided to enter the display by a polarized beam splitter, with no consideration being paid to an application thereof to a direct view type. No details are given about the setting of .vertline.d/p.vertline. where p is the natural pitch of the liquid crystal and d is the cell thickness, and optimum .DELTA.n.times.d where d is the cell thickness and optimum .DELTA.n is defined as ne-no.
As for the portable information device, a touch panel (pressure sensitive input device) is a useful input tool as well as conventional keyboards. Especially, to input languages that require the conversion of data fed through a keyboard such as the Japanese language, the touch panel acts not only as a pointing device, but also, more and more generally, as a direct input device for handwritten characters thanks to the enhancement of information processing capability and the development of software. This input method is realized by placing an input device before a display.
Since the reflective-type liquid crystal display device utilizes the reflected light to realize display, the touch panel cannot impair display performed by the reflective-type liquid crystal display device placed thereunder. Japanese Laid-Open Patent Application No. 5-127822/1993 (Tokukaihei 5-127822/1993) discloses that the low reflection processing is done by overlapping a quarter-wave plate and a polarizer plate over a touch panel.
However, the liquid crystal display device disclosed in Japanese Laid-Open Patent Application No. 5-548733/1980 needs a quarter-wave plate disposed between the liquid crystal layer and the reflective plate; therefore basically it is difficult to form a reflective film on an inner side of the liquid crystal cells and the liquid crystal display device is not suitable for high resolution, high fidelity display.
In addition, even if a satisfactory level of performance has been successfully realized to be commercialized as a reflective-type liquid crystal display device, the visibility deteriorates greatly when used with a touch panel.
The deterioration in visibility of a transparent liquid crystal display device and other illumination-type displays used with a touch panel can be easily solved by removing, or changing the direction of, the glare image due to the light source (e.g. a light on the ceiling) that is a primary cause for the light reflected by the touch panel. However, in the reflective-type liquid crystal display device, the light source, although causing the light reflected by the touch panel, is the display light source for the display device as well. So the above solution is not applicable.
Therefore, solving the problem of poor visibility is the key to the realization of a useful low-power-consuming portable information device as well as to the realization of a reflective-type liquid crystal display device. The arrangement of a touch panel disclosed in Japanese Laid-Open Patent Application No. 5-127822/1993 is advantageous in that the combination of the polarizer plate at the quarter-wave plate prevents reflection, but an ordinary quarter-wave plate inevitably worsens visibility at certain wavelengths in the visible region of the spectrum.
If the display device (e.g. a white-tailor-type Guest Host liquid crystal display device with pigment added to the 360.degree.-twisted liquid crystal) with practically negligible polarization characteristics is used beneath the touch panel, the reflection efficiency takes a half of the value obtained when no touch panel is used, due to transparency of the polarizer plate before the touch panel.
If the display beneath the touch panel uses linearly polarized light to perform display (e.g., TN and STN liquid crystal display devices having another polarizer plate between the touch panel and the liquid crystal cell), the reflection efficiency takes a half of the value obtained when no touch panel is used. In addition, since the phase difference of the quarter-wave plate depends on the wavelength of light, and the quarter-wave plate is sandwiched by the polarizer plates, tones are changed.
In any of the above cases, the brightness is not satisfactory, the touch panel cannot be suitably combined with a reflective-type liquid crystal display device with no means to improve brightness, such as a backlight. In other words, the touch panel disclosed in Japanese Laid-Open Patent Application No. 5-127822/1993 raises a need for a further improvement of the reflection prevention function, and the Patent Application dose not disclose a suitable arrangement to utilize the external light entering the touch panel for the reflective-type liquid crystal.
Typically, the present liquid crystal display device adopts a planar alignment mode where liquid crystal molecules are aligned parallel to the substrate when no voltage is applied.
On the other hand, a vertical alignment mode where liquid crystal molecules are aligned vertical to the substrate when no voltage is applied is adoptable to perform display. When a liquid crystal display device operates in a so-called normally black mode, in which dark display is effected using the vertical alignment when no voltage is applied across the liquid crystal layer, a darker and better black display can be effected in comparison to the planar alignment mode, and therefore the display contrast is improved. Since the liquid crystal layer does not cause birefringence with light transmitted in the normal direction of the liquid crystal layer through the liquid crystal layer when no voltage is applied across the liquid crystal layer, the normally black mode has a characteristic that the arbitrariness in the liquid crystal layer thickness (liquid crystal cell gap) is greater.
In other words, in this case, the liquid crystal layer thickness can be larger than the conventional planar alignment mode, the margin is wider for variations in the cell gap, and in any case good black display becomes possible. It is further known that if vertical alignment normally black mode is used, there is little negative effect from light leaking due to spacer beads for maintaining the cell gap of the liquid crystal layer.
Despite the characteristics described above, it has been considered that the vertical alignment mode is difficult to realize a uniform alignment state and can find few applications in industry. Especially, it has been considered that the vertical alignment mode is difficult to effect stable alignment when an electric field is applied across the liquid crystal layer and the alignment of the liquid crystal layer molecules is deformed from the vertical direction.
Recent publications including Enclosure Electrode Method (Japanese Laid-Open Patent Application No. 7-64089/1995 [Tokukaihei 7-64089/1995]) and Multidomain Method (Nikkei Microdevice, January 1998 Issue, page 136) disclose that a useful method to provide a solution to this problem and effect a vertical alignment mode in an industrially more stable manner is to change the shape of the substrate by a method of changing the direction of the electric field or an insulating structure within an area of approximately one pixel of a display.
Methods to slightly incline (tilt) the initial liquid crystal alignment with respect to the normal direction of the substrate using a rubbing method (Japanese Laid-Open Patent Application No. 62-299814/1987 [Tokukaisho 62-299814/1987]) or an photo-induced alignment method (Control of Liquid Crystal Alignment Using an Optically Active Polymer Film, by Mr. Yasushi IIMURA, Tokyo University of Agriculture and Technology, First JLCS-ALCOM Joint Conference) are disclosed as means to obtain uniform alignment with respect to the vertical alignment over the entire panel when voltage is applied. It is expected that the methods will provide other solutions for the problem than the creation of microdomains with the previously mentioned methods.
However, the enclosure electrode method and the multidomain method are needed to make the upper and lower substrates sandwiching the liquid crystal layer in a precise and complex structure, resulting in an increased number of manufacturing processes, and inevitably the need to make a domain having a period of repetition equal to the size of a pixel. This means that the period of repetition of an insulating component or the structure of an enclosure electrode that determine the period of repetition of the domain cannot be designed freely due to the need to effect stable alignment, and should be designed to fall in a certain range.
The size of the pixel, determined by the displayed content, the size of the displayed image, and the display capacity, ranges from the minimum value of about 10.mu.m to the maximum value of about 1mm, the former being a hundred times the latter. The shape of the pixel also varies greatly. However, there is no report on whether the period of repetition of the domain is effective with the hundred-time-wide range and various pixel shapes, and it is not yet confirmed whether the period of repetition of the domain is applicable to all the liquid crystal display devices adopting vertical alignment. Moreover, there is a structure of an insulating body provided to, or inside of, the electrodes of both substrates, which reduces the voltage applied across the liquid crystal layer and therefore increases the driving voltage.
As described so far, since there are problems to be solved to achieve a stable vertical alignment through the electrode structure or substrate structure, it is considered that the above-mentioned rubbing and photo-induced alignment methods that are free from such problems are preferable. However, as for these methods, there is no specific description on the conditions of the tilt angle of the liquid crystal molecules with respect to the normal direction of the substrate, no insight is obtained about to what extent the alignment should be inclined to take effect.
Accordingly, the inventors of the present invention have diligently worked and sorted out specifically that a small tilt angle leads to three problems.
First when an electric field is applied to effect display with a liquid crystal display device adopting a vertical alignment and a small tilt angle, a disturbance in liquid crystal alignment properties due to application of an external stimulus, such as pressure, to the substrate results in a persistent alignment disturbance or in an alignment defect such as a domain wall. This is an especially serious problem to hand-writing input utilizing a touch panel or another device that is useful input device for portable information devices.
Secondly a small tilt angle results in the optical transparency and reflectance showing slow response and change with the application of an electric field.
Thirdly a small tilt angle results in an increased driving voltage in comparison to a large tilt angle.